London-based lawyer Hani al-Sebai said on Tuesday that Sarwat Salah Shehata is currently being held by Tehran with around 350 Arab and Egyptian Islamists.
Shehata has been sentenced to death twice by Egyptian courts, and is on America's most wanted list.
"Egypt recently sent an interior ministry delegation (to Iran) to identify Egyptian prisoners, before making an official request for their extradition," said Sebai.
However, his claims could not be corroborated by the Egyptian or Iranian governments.
Shehata left Egypt in 1991 for Afghanistan, where he worked with fellow Egyptian Ayman al-Zawahri, Usama bin Ladin's right-hand man and number two in the al-Qaida network.
But Sebai told Arab daily al-Hayat that Shehata is not linked to the group, accused by the United States of carrying out the September 11 attacks.
"Egypt recently sent an interior ministry delegation (to Iran) to identify Egyptian prisoners, before making an official request for their extradition"
He said Shehata led a group of fighters who broke away from Zawahri and bin Ladin in 1998.
It is unclear when Shehata was arrested, but he has lived in Iran for two years.
"Several Arab militants, arrested since 11 September 2001, have already been extradited to Europe or their home countries, frequently via Syria," Sebai said.
"The Iranian authorities have set up a special detention centre for militants and their families," he added.
Diplomatic sources and the Arab media say Iran is holding Saad bin Laden, a son of the Al-Qaida leader; Zawahri; another Egyptian Saif al-Adel, who is Al-Qaida's number three; and Sulayman Abu Ghaith, the network's spokesman.
Dubai-based satellite television channel al-Arabiya broadcast an audiotape on Sunday purported to have been made by Zawahri.
The al-Qaida number two vowed to take revenge if any prisoners held at the US military base at Guantanamo Bay are sentenced to death.
Tehran said on Monday that it had decided to keep secret the identities of al-Qaida suspects in its custody "for security reasons."
It also ruled out a trade-off with Washington for members of the Iranian armed opposition People's Mujahidiin.